This invention relates to improving the tolerance of composite laminate structures to damage and/or delamination, resulting in weakening of the structure and/or to reducing the possibility of such damage and/or delamination. In particular, but not exclusively, the invention relates to protecting an end surface of a composite laminate structure of an aircraft component from impact damage.
Many aircraft components are made from composite laminate materials. For example, a stringer in an aircraft wing, stiffeners and spar structures, may all be in the form of composite laminate structures. While the structures may be strong when loaded in the way in which they have been designed to be loaded, they may be vulnerable to impact damage, which may result in delamination occurring. Delamination may result in the structure weakening and eventually failing. Particularly vulnerable are the planar end faces of composite laminate structures where an impact occurs, particularly if the impact is end-on (i.e. when the impact is in a direction perpendicular to the end face and parallel to the plane of the laminates that make up the structure). It will be understood that the planar end face is the surface at which the layers of the composite laminate structure terminate and that the normal axis of the end face is typically parallel to the direction in which the composite laminate structure extends from the end face. If an impact has a significant component of force in the end-on direction the impact may be capable of damaging the bonding between the laminate's layers comprising the composite laminate structure and causing, or adding to the effect of, delamination. Such impacts may be as a result of workmen dropping tools whilst working on the structure of an aircraft, during maintenance for example when the end surfaces of the composite laminate structures are exposed in a way they are usually not. A further way in which the composite laminate structures may be damaged is as a result of wear over time, such wear for example resulting from workmen sitting, or standing, on the structure or contacting the structure directly or by means of tools, or other equipment. It is for example common for a workman to rest tools or toolboxes on the end surface of such structures while working on the aircraft. The planar end face of a composite laminate structure is typically exposed, at least during maintenance, to direct wear and direct impacts as described above.
When designing an aircraft, factors that affect the strength and other mechanical properties of composite laminate structures of the aircraft, such as those factors described above, are taken into account. Thus, factors such as those described above typically mean that the composite laminate structures need to be provided with extra strength, resulting in an increase in size and weight, both of which are undesirable in aircraft design.